WO 2010/063576 A1 discloses a jetting device of this type which is used for jetting molten metal such as copper, silver, gold, and the like. The magnetic field generator creates a magnetic field that extends at right angles to a flow direction of the liquid when the liquid flows to the nozzle. The electrodes are arranged to create an electric current that is normal to both the magnetic field and the flow direction of the liquid. As a consequence, the electrically conductive liquid is subject to a Lorentz force that accelerates the liquid towards the nozzle, so that, when the electric current is applied in the form of a pulse sequence, droplets of the molten metal are jetted out from the nozzle.
In such a device, the jetting stability may be compromised by small impurities and in particular by small gas bubbles in the liquid in the fluid chamber.
It is an object of the invention to provide a jetting device that permits a maintenance operation for maintaining a high jetting stability.
In order to achieve this object, the magnetic field generator is arranged to create a rotating magnetic field in the fluid chamber.
When a rotating magnetic field passes through a solid or liquid body of an electrically conductive material, the magnetic field induces eddy currents in the conductive medium, with the result that an electromagnetic interaction between the medium and the magnetic field causes the medium to co-rotate with the magnetic field. Thus, by creating a rotating magnet field in the fluid chamber, it is possible to create a vortex flow of the electrically conductive liquid in the fluid chamber. As a consequence, centrifugal forces urge the liquid outwardly against the peripheral wall of the fluid chamber, and gas bubbles that may have adhered to that wall will be detached from the wall and displaced inwardly where they may float up to the surface of the liquid, so that they are eventually removed from the fluid chamber. In this way, whenever it is observed or feared that the jetting stability of the device may be compromised by gas bubbles, these bubbles can easily be removed by switching the device temporarily into a maintenance mode in which the rotating magnet field is created. Since the device has a magnetic field generator, anyway, for producing the Lorentz force that accelerates the liquid towards the nozzle, only a little modification of the existing magnetic field generator is necessary for enabling this generator to create a rotating magnetic field.
Gas bubbles may be present in the fluid chamber, for example due to the presence of dissolved gas in the metal, or due to gas entering the fluid chamber via the nozzle. The presence of gas bubbles in the fluid chamber may hamper the jetting process and is therefore disadvantageous.
In an embodiment, the jetting device may comprise a detection unit for detecting presence of a gas bubble in the fluid chamber. Such detection unit is described in detail in EP 2547527, which is herein incorporated by reference. Such detection unit may be an electrical signal detecting unit for detecting a resulting electrical current, which resulting electrical current is induced by a residual pressure wave in the part of the conductive fluid positioned in the magnetic field, thereby obtaining a detection signal. When a droplet of fluid is ejected, differences in pressure are generated throughout the fluid and a pressure wave is created. Due to inertia, the pressure wave does not disappear momentarily as soon as the application of the electrical current is stopped, but will gradually fade in the course of time. The residual pressure wave as a function of time will depend, amongst others, on the acoustic behavior of the fluid chamber. The pressure wave will generate motion in the conductive fluid, which will fade in the course of time, like the pressure wave. The motion in the conductive fluid generates a force. Thus, after ejection of a droplet, a force is generated in the fluid. Since the conductive fluid is positioned in a magnetic field, an induced current ({right arrow over (I)}) is generated in the fluid, because of the relation {right arrow over (F)}={right arrow over (I)}×{right arrow over (B)}. By measuring this current, the acoustics in the actuation chamber may be monitored. By monitoring the residual pressure wave, the presence of a gas bubble can be detected. When a gas bubble is detected, a rotating magnetic field may be generated in the molten metal to remove the gas bubble. A control unit may be provided that is operatively connected to the detection unit. The control unit may control the magnetic field based on an input signal of the detection unit.
The rotating magnetic field may be generated using a plurality of magnetic field generating units. A magnetic field generating unit may comprise a magnet and/or a magnetic field concentrator. Preferably, the rotating magnetic field may be generated using at least two pairs of magnetic field generating units.
Useful details and further developments of the invention are indicated in the dependent claims.
In order to create the rotating magnetic field, the magnetic field generator may simply be rotated about the axis of the fluid chamber and the nozzle. In another embodiment, the magnetic field generator may be arranged to create at least two non-parallel magnetic fields the respective strengths of which can be modulated such that a torque is applied to the electrically conductive liquid. For example, two electromagnets may be provided for creating magnetic fields that are orthogonal to one another and these two electromagnets may be driven with sinusoidal currents with a phase difference of 90°. The result will be a magnetic field that rotates continuously and with constant angular velocity. It is however not prerequisite that the magnetic field rotates continuously and with constant angular velocity. In other embodiments, the magnetic field may be switched on and off intermittently and, while it is switched on, may be rotated through an angle of less than 360°.
In jet another embodiment, the magnetic field generator may comprise a combination of electromagnets and movable permanent magnets. For example, when permanent magnets are used for creating a stationary magnetic field in a first direction for producing the Lorentz force during the jetting operation, an electromagnet may be used for modulating that field, and another electromagnet may be used for creating a magnetic field in a second direction normal to the first direction.
The magnetic field generator may be controlled such that the angular velocity of the rotating magnetic field is increased gradually in order to gently accelerate the vortex flow of the liquid in the fluid chamber.